The field of the invention is rotary drum vacuum washers, e.g., filters, used in the pulp and papermaking industry to form a mat of wood pulp and cleanse the mat of filtrate. In particular, the invention relates to the filtrate drainage systems for vacuum drum washers.
Vacuum washer drums remove pulping liquors and other liquids from pulp. A vacuum washer has a large rotating cylindrical drum that sits partially in a vat of pulp and liquor. The following references the drum as it rotates in a clockwise direction. As the drum surface rotates through the vat, e.g., 3:00 to 9:00 drum positions, a pulp mat forms on the wire screen surface of the drum. The screen prevents pulp from flowing into drainage passages in the drum. A suction is applied to the drum surface through the drainage passages. The suction pulls the liquor through the wire screen on the drum surface and causes a pulp mat to form on the surface. The suction draws the wash liquid through the mat and into the drainage passages. As the drum surface with pulp mat rotates up and out of the vat from the 9:00 to 12:00 position, water is sprayed on the pulp mat to remove cooking liquor from the pulp. The water and liquor (but not pulp fibers) pass through the wire screen and flow into the drainage passages. The water and liquor in the drainage passages is referred to as “filtrate”. The washed pulp mat is removed from the drum surface, at about the 2:00 to 3:00 drum position, before the drum surface rotates down into the vat. The drum surface rotates back into the vat to pickup another pulp mat.
The drainage passages are internal to the drum and typically include channels immediately behind the wire screen surface and deck extending along the entire length of the cylindrical wire screen surface. The channels conventionally drain into radial passages at the end of the drum (“end draining drum”) or into a conical array of drain tubes extending from a center annular drain behind the wire screen and deck (“annular center draining drum”). The drain tubes of the annular center draining drums extend from the drum surface at the center of the drum to an end of the drum. The conical array of drainage tubes discharge through an annular disc tube sheet at an end of the drum and into a V-trunnion that caps the tube sheet. The tube sheet and V-trunnion have relatively large diameters, e.g., 50 inches to 60 inches (127 cm to 152 cm), to accommodate a large number of drainage tubes, e.g., 30 to 36 tubes, that each have a relatively large diameter of, for example, 6 inches (15 cm).
The radial end drain tends to be inexpensive to manufacture and maintain, as compared to the center draining drum. The radial end drain has difficulty in draining filtrate from the far end of long drums, such as where the drum length exceeds 20 feet (6 meters). The annular center drain is typically used for longer drums, e.g., longer than 20 feet (6 meters), but is expensive to manufacture and maintain. The annular center drain is expensive, in part, because the V-trunnion is a large device having intricate drain passages that direct filtrate from each of the tubes to an axial drain. There is a long felt need for a less expensive filtrate drainage system for vacuum washers having long drums.